Solid golf balls are golf balls other than wound golf balls which contain a center wound with elastic rubber thread, including one-piece solid golf balls which are produced by integrally molding, and two-piece solid golf balls which are composed of a solid core and a cover covering the solid core. The solid core of the two-piece solid golf balls may be composed of two or more layers, which are often called multi-piece solid golf ball. Any solid golf balls integrally or partially have resilient portions which are prepared by vulcanizing and molding a rubber composition.
In order to improve impact resilience of the resilient portion, many investigations have been made on ingredients to be formulated into a rubber composition, vulcanizing conditions and the like.
Japanese Kokai (unexamined) Publications 275356/1988 and 89750/1987 suggest to employ a polybutadiene which has a higher Mooney viscosity than a polybutadiene conventionally used for golf balls. Since the suggested polybutadiene contains high molecular weight fractions in a large quantity, the impact resilience and durability of the solid golf balls are improved.
Although it is considered to employ a untra-high molecular weight polybutadiene which has very high molecular weight far from the convention polybutadiene, it has some disadvantages in processability. Thus, the ultra-high molecular weight polybutadiene is very difficult to mix with other ingredient.